I am a member of a number of Stack Exchange sites. Recently I seem to have gotten logged out from a number of them (Server Fault, Super User) and I can't figure out a way to log in without scrounging in my password manager and copy/pasting e.g. my Stack Overflow password into them.

When you log into any Q&A site on the stackexchange.com domain, you
will be automatically logged into all other Q&A sites on the
stackexchange.com domain + stackexchange.com itself.

Area 51 is excluded from this. stackoverflow.com, superuser.com, and
other Q&A sites with their own domains will still use the old flow.

[…]

But is there now (in January 2019) no longer a way to just be signed in everywhere? Or am I just noticing this because of item number 2 in the quoted text — if so, what exactly is "the old flow"? What's the easiest way to keep logged into everything Stack Exchange [without staying logged into Google or Facebook shudder].

Do you allow third-party cookies? Mind sharing which browser and OS you're on. Any plugins/extensions running? If you fancy debugging this yourself you might want to read my answer here.
– reneJan 10 at 21:11

I am currently using Chrome, set to block third party cookies (and with the "uBlock origin" extension). Hoping to switch to Brave soon which I anticipate will be similar. Great writeup, appreciate it!
– natevwJan 10 at 21:48

Ok, so the SSO is intended to happen whenever you're on a logged-in site, by giving the cross-origin SE properties a chance to set a [considered third-party] cookie. That's great for some users… but I guess there's no auth flow I can use that provides a fallback when such cookies are ignored?
– natevwJan 10 at 22:01

That is correct, you need to allow third-party cookies. See also what is said in Stage 3 of the Universal Auth post: Safari users and users who reject third-party cookies will need to sign in separately to every second-level domain (e.g. stackexchange.com, stackoverflow.com, etc).. I guess Adam hoped to find a solution, hence the stage 3 step for the implementation but in the end none of that has worked out. No fallback exists.
– reneJan 11 at 5:52